This invention generally relates to X-ray apparatus and, more particularly, to a system for automatically regulating the emission of the X-ray tube during its operation as well as controlling the warm-up process.
In the operation of X-ray apparatus, it is often very desirable to maintain the X-ray tube current at a precise level because X-ray emission is directly related to the amount of current flowing between the anode and cathode of the tube. Precise control of the amount of X-ray emission is of course desirable because it is important to have the same X-ray exposure each time the apparatus is set by the equipment operator to make that exposure.
Conventional approaches to the problem of regulating X-ray tube current rely upon the concept of regulating the voltage applied to the filament circuit of the X-ray tube to control the filament current. Such voltage regulators have several drawbacks in that the amount of X-ray tube current is proportional to the temperature of the cathode filament which, in turn, is a function of both the resistance of the filament and the amount of current passing therethrough. As the filament ages, its resistance changes because a portion of the filament evaporates. Moreover, age and temperature affect the values of the circuit components supplying the voltage/current to the filament. As a result, periodic readjustment of the apparatus is required to compensate for these changes.
Another problem with conventional X-ray apparatus relates to the amount of time required to prepare the apparatus for operation and the manner in which it is accomplished. It is desirable in some procedures such as "spot filming" to change from a low emission level (fluoroscopy) to a high emission level (radiography) as quickly as possible. To heat the filament quickly it is necessary to supply a very large amount of current during the first 0.2 seconds when the filament is cold and very low in resistance. Voltage regulated systems with fixed source impedances cannot supply optimum amounts of current during the cold start, and therefore cannot start as fast as a current regulated system, such as hereinafter described.